1. Field of the Invention
The present invention relates to the use of femtosecond lasers to treat or expose glass, to form a pattern, such as a waveguide. The method can be used to form optical devices, such as light guiding structures. The invention also relates to waveguides formed in glass.
2. Description of Related Art
Optical devices such as optical waveguides and Bragg diffraction gratings are widely known in the telecommunications field. In an optical waveguide, a higher refractive index core surrounded by a lower refractive index cladding can transmit a large amount of optical information over long distances with little signal attenuation. The optical waveguide fiber is the prototype device of this type. The fiber is produced by a method that, by virtue of its fabrication, gives the proper waveguiding structure. A Bragg grating is another type of optical device that can be used to isolate a narrow band of wavelengths from a broader signal. The most common materials used commercially in telecommunications applications of light guiding devices are doped silica-based compositions.
It is known that pulsed laser sources can be used to effect both index changes and to produce physical damage in glass. With regard to the former, the use of pulsed UV radiation sources for writing Bragg gratings is known. Recently, a xe2x80x9cdirect-writexe2x80x9d laser method of forming optical waveguides within a glass volume that is transparent to the wavelength of a femtosecond laser has been disclosed. In this method, a 120 fs pulsed 810-nm laser is focused within a polished piece of germania-doped silica as the glass is translated perpendicular to the incident beam through the focus. Increases in refractive index on the order of 10xe2x88x922 were reported for a specific condition in which the focus was scanned ten times over the exposed area. See K. Hirao and K. Miura, J. Non-Crystl. Sol 235, pp. 31-35, 1998.
One potential problem with a direct write process of forming waveguides in bulk glass using short-pulse focused lasers is that of over-exposure. Irradiation with too much energy can lead to physical damage in the glass. Physical damage results in undesired attenuation of optical signals transmitted through the glass.
Another problem in direct write methods of making optical structures relates to the trade-off between the stability of the writing device, e.g., the laser, and the energy necessary to induce the desired refractive index change in the substrate material.
A femtosecond laser system is one generating a train of optical pulses with temporal widths of 10-200 fs. The femtosecond laser system is generally a femtosecond mode locked oscillator, referred to herein as a femtosecond laser. The repetition rate and the energy per pulse are a function of the particular system. In general, the systems include an oscillator (a system that delivers energy) from which the pulse train is developed. The maximum energy per pulse is about 1-10 nJ. The repetition rate, which is essentially the round trip time in the cavity, can be very high, on the order of 100 MHz. If more energy is needed, an amplifier section is added. Here, the pulse is broadened to reduce the intensity, amplified, and then the pulse is compressed. One can achieve mJ levels of energy but at the expense of the repetition rate. Depending on the specific amplification scheme and ultimate pulse width, the repetition rate can range from 1 kHz to 250 kHz. Another approach is the so-called xe2x80x9ccavity-dumpedxe2x80x9d approach. These three approaches are summarized in the following Table I.
The table above illustrates the operational trade-offs as a consequence of how the laser is configured. While it is relatively easy to obtain a 100 MHz repetition rate using the oscillator mode when the pulse energy is less than 10 nJ, at the xcexcJ level of energy the repetition rate is traded off and drops to the several kHz range. Mode quality, which is qualitatively described by the temporal and spatial integrity of the beam, is relatively poor in the amplified system and improves when the oscillator is used. Similarly, the overall stability of the laser is found to be more robust in the oscillator case. These parameters turn out to be of practical importance in direct-write methods of making optical devices where one needs to control the pointing stability of the laser beam in order to write closely spaced optical structures in the substrate, such as diffraction grating lines.
To make the femtosecond laser direct-write method practical, large changes in the refractive index, such as  greater than 10xe2x88x923, of a material should be achieved in a reasonable amount of writing time. The formation of laser-induced physical damage should also be avoided. There is a need to find materials that can meet the requirements of large refractive index change, without physical damage to the material. It is also a desire to be able to use the relatively low energy of the oscillator, without needing an amplifier, in producing optical devices, such as waveguides.
Moreover, there continues to be a need for a practical direct write method of creating optical devices having a sufficiently increased refractive index at an acceptably high write rate. Such a method could be used to write continuous light-guiding waveguide patterns connecting any two points within a continuous block of a suitable material, or make other optical devices, such as Bragg gratings.
In accordance with these and other needs, there has been provided according to the invention a method of writing a pattern in a bulk glass substrate having an absorption edge (xcexg), including focusing a pulsed laser beam having a wavelength (xcexex) such that xcexg∠xcexex∠2xcexg at a focus within said substrate while translating the focus relative to the substrate along a scan path at a scan speed effective to induce an increase in the refractive index of the material along the scan path relative to that of the unexposed material while incurring substantially no laser induced breakdown of the material along the scan path.
In accordance with these and other needs, there has also been provided according to the present invention, a method of writing a pattern in a bulk glass substrate selected from the group consisting of borosilicate, lead, and sulfide glass, including focusing a pulsed laser beams at a focus within said substrate while translating the focus relative to the substrate along a scan path at a scan speed effective to induce an increase in the refractive index of the material along the scan path relative to that of the unexposed material while incurring substantially no laser induced breakdown of the material along the scan path.
In accordance with the present invention, there has also been provided an optical device, such as a waveguide,.formed from one or more of borosilicate, lead, and sulfide glass that has had an induced refractive index change.
Further objects, features, and advantages of the invention will become apparent from the detailed description that follows.